Protecting completion and production equipment is of utmost concern during acidizing. The higher the temperature, the more difficult it is to protect metal against corrosion, and the required inhibitor loadings increase with temperature, resulting in greater likelihood of formation damage. In addition, the protection times are reduced dramatically, which can potentially limit well stimulation treatments (for example, fluid volumes caused by pump time limitations). These problems become increas-ingly severe in formations with bottomhole temperatures greater than 250°F (120°C).
Even if adequate corrosion protection can be achieved during stimulation, post-treatment production of chlorides associated with the injected HCl-based acids remains a prob-lem. There is increasing concern when the wellbore contains high-alloy metals, such as stainless and duplex steels, which are susceptible to hydrogen embrittlement and chloride stress cracking. When combined with the possibility of erosion corrosion caused by high production rates, acidizing high- pressure, high-temperature (HPHT) wells poses risks.
A combination of organic acids (acetic and formic) can be used instead of hydrochloric acid (HC1) to minimize corrosion problems in high-temperature applications. The blends are designed so that the dissolving power is equivalent to HC1 with significantly reduced corrosion rates and the absence of Cl' ions. Some of the gelling agents developed for HC1 yield higher viscosities in organic acids on an equivalent polymer-loading basis. The end result is an organically based, high-temperature acid system that uses existing technology and is technically and economically more attractive than an HCl-based system.
The example application includes the use of an organic acid blend to effectively stimulate the 350°F (175°C) Arun limestone formation in Indonesia. A total of 17 large-scale acid fracs and a similar number of large matrix-acidizing treatments have been performed over the past 3 years. Rheology and corrosion data will be presented with the details of the treat-ment procedures and production responses.